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Published online by Cambridge University Press: 11 June 2019
This is a copy of the slides presented at the meeting but not formally written up for the volume.
Reflection high-energy electron diffraction (RHEED) is one of the most robust and widespread techniques used for in-situ monitoring during molecular beam epitaxy (MBE) growth. Thus, all MBE systems have an electron gun allowing additional electron-beam stimulated in-situ characterizations. At WVU we are developing two such techniques, spectral analysis of cathodoluminescence (CL) in wide bandgap semiconductors and reflection high-energy electron diffraction-total reflection angle x-ray spectroscopy (RHEED-TRAXS) for in-situ composition monitoring and control.A pressing issue remaining for epitaxial growth is real-time compositional control to a high level of accuracy. For many materials, such as multi-element nitrides and oxides with unity sticking coefficients, it would be extremely beneficial to monitor the composition to a fraction of a monolayer. This technique needs to be both element-specific and surface-sensitive. RHEED-TRAXS is a leading contender as such a technique. The electron beam from a RHEED gun impinges on the sample at a small angle of incidence approximately equal to the critical angle for x-ray reflection. This geometry ensures that the measurement is extremely surface sensitive. This technique can be used to obtain both structural information, via RHEED, and chemical information, via x-ray detection. We are currently developing a compact RHEED-TRAXS using a state-of-the-art Si P-intrinsic-N (PIN) photodiode technology. We have used this system to investigate Ga and In coverage during the growth of GaN, and have observed Ga bi-layer evolution during growth, Mg destabilization of the Ga wetting layer, and significant In surface segregation. We are also investigating the in-situ, real-time composition measurements in complex oxide systems such as YMnO3 with promising initial results.In-situ cathodoluminescence (CL) occurring during RHEED is a strong candidate to determine the growth temperature and alloy composition for wide bandgap semiconductors. CL is easily detected up to and beyond typical growth temperatures for GaN and InGaN, accurately and reproducibly determining sample temperature during growth. Room CL measurement at room temperature can also be used as a means to check the quality of the substrate by comparing intensities of the GaN band edge energy peak and defect peaks. We have performed a detailed study of the factors influencing high temperature CL, and find the reproducibility of CL data and ability for fast CL scanning provide strong advantages for use in the growth of GaN films. CL could also be observed during growth using a ccd camera. This could be used to see temperature inhomogenaities, and potentially to map alloy composition fluctuations. Using tunable narrow bandpass optical filters, we can obtain a spatial/spectral map of sample CL. We will present CL images of samples at differing temperatures.This work was supported by the AFOSR MURI F49620-03-1-0330 and by ONR Grant N00014-02-1-0974.